Best bet is to study known examples of his signature really get into those details. Make a screen collage of true exemplars and stick the one you want in with them and say does this fit with the rest etc. You gotta step up your game and then you can find a good one out there. Obviously stay away from bad sources but I don't really go by sources I use my eyes and get opinions if im not quite sure... sometimes even good sources have a fake and dont even know it...
Also stay away from questionable signatures to begin with. Its hard to convince anyone its real even if its stamped with psa approval if its just a squiggly line etc. It's gotta have good eye appeal too for resale value...
Hello, RACC (real autograph collectors club) on Facebook has quite a few people that have authentic Clint Eastwoods for sale. If you wish to join mention Ian has recommended you so I can see its you to add you.